1. Field of the Invention
The present invention relates to a stepping motor, and specifically to a low-profile stepping motor having its dimension reduced in an axial direction.
2. Description of the Related Art
Recently, in accordance with enhanced performance of various devices such as printers, facsimiles and floppy (Registered Trademark) disk drives, a stepping motor is required to be higher in torque, and lower in cogging torque and vibration. Under the circumstance, a PM (permanent magnet) stepping motor such as is disclosed in Japanese Patent Publication No. Hei 10-127024 has been extensively used to meet the requirements.
FIG. 1 is a partly cutaway perspective view of a conventional PM stepping motor. As shown in FIG. 1, the PM stepping motor generally comprises: a stator assembly 6 composed of two stator units 6A and 6B; a rotor assembly 13; a face plate 1 punched out of a stainless steel plate; and a front bearing 2 made of oil impregnated alloy. The stator units 6A and 6B each comprise: stator yokes 3a and 3b each punched out of a soft magnetic steel plate and each having a plurality of pole teeth 10 bent up along its inner circumference; and a coil 4 formed such that a copper wire with polyurethane insulation is wound on a bobbin 5 housed in the stator yokes 3a and 3b, and are attached back-to-back to each other. The rotor assembly 13 comprises: a rotary shaft 8; a cylindrical magnet 9 for magnetic field, having a plurality of magnetic poles at its circumferential surface, and having the circumferential surface opposing the pole teeth 10 of the stator yokes 3a, 3b; and a sleeve 12 to fasten the cylindrical magnet 9 and the rotary shaft 8 together such that the rotary shaft 8 is press-fitted into the sleeve 12, and the sleeve 12 with adhesive applied thereon is inserted into the cylindrical magnet 9. The rotor assembly 13 thus structured has its outer circumference (of the magnet 9) magnetized for a predetermined number of magnetic poles, and is rotatably disposed inside the stator assembly 6 such that the rotary shaft 8 is supported by the front bearing 2 and a rear bearing (not shown in FIG. 1), and that the magnetic poles oppose the pole teeth 10 with a slight gap therebetween.
The stator unit 6A is structured such that the pole teeth 10 of the stator yoke 3a mesh with the pole teeth 10 of the stator yoke 3b equidistantly from each other with a shift by an electrical angle of 180 degrees (half pitch). The stator unit 6B is of the same structure as the stator unit 6A. The stator units 6A and 6B thus structured are joined back-to-back to each other by molding with polymeric material or by plasma welding such that their respective pole teeth 10 and 10 are shifted from each other by an electrical angle of 90 degrees, whereby the rotor assembly 13 always starts its rotation in the same direction.
Recently, since the devices incorporating PM stepping motors are required to be downsized and lower profiled, the PM stepping motors are also required to be further lower profiled. The conventional PM stepping motor, however, is composed of two stator units joined to each other back-to-back thus making it extremely difficult or almost impossible to further reduce its profile dimension. And the conventional PM stepping motor uses four discrete stator yokes, which inevitably increases the number of the components and makes alignment troublesome.
The present invention has been made in view of the above circumstance, and it is an object of the present invention to provide a low-profile stepping motor which is of a simple structure and which has its dimension drastically reduced in the axial direction.
In order to achieve the object, according to a first aspect of the present invention, a low-profile stepping motor comprises: a stator assembly composed of first and second stator units; a rotor assembly; and a spacer block. The first stator unit includes: a first upper stator yoke having a plurality of pole teeth which are formed along its semicircular inner circumference; a first lower stator yoke having a plurality of pole teeth which are formed along its semicircular inner circumference and which mesh with the pole teeth of the first upper stator yoke; and a first coil unit fixedly sandwiched between the first upper and lower stator yokes. The second stator unit includes: a second upper stator yoke formed as one piece integrally with the first upper stator yoke, and having a plurality of pole teeth which are formed along its semicircular inner circumference; a second lower stator yoke formed as one piece integrally with the first lower stator yoke, and having a plurality of pole teeth which are formed along its semicircular inner circumference and which mesh with the pole teeth of the second upper stator yoke; and a second coil unit fixedly sandwiched between the second upper and lower stator yokes and arranged horizontally flush with the first coil unit. The rotor assembly is rotatably disposed in a circular open space defined by the pole teeth of the first and second stator units. And the spacer block is disposed in the stator assembly and has openings shaped to surround the first and second coil units, and the pole teeth. With the structure described above, while the stepping motor can be significantly reduced in thickness, the entire assembly can be made rigid even if the stator yokes are made of a thin steel plate for downsizing, resin molding for a gap space between the intermeshing pole teeth can be performed using only a molding die set inside the pole teeth, and magnet wires of the coil units are prevented from breaking.
According to a second aspect of the present invention, in the low-profile stepping motor of the first aspect, the spacer block is of a single piece structure. Consequently, the spacer block keeps readily and surely the upper and lower stator yokes equidistant from and in parallel to each other.
According to a third aspect of the present invention, in the low-profile stepping motor of the first or second aspect, the spacer block is formed of resin. Consequently, an insulating spacer block can be produced easily.
According to a fourth aspect of the present invention, in the low-profile stepping motor of any one of the first to third aspects, the spacer block has, on its side, a plurality of hooks for securely retaining lead wires leading out from the first and second coil units. Consequently, the lead wires are prevented from accidentally getting unhooked, thereby providing high reliability.
According to a fifth aspect of the present invention, in the low-profile stepping motor of the fourth aspect, the hooks are shaped like an L-letter in cross-section with their shorter bar sections joined to the side of the spacer block and are disposed in a line such that the shorter bar sections are positioned alternately at the opposite ends to one another. Consequently, the lead wires are surely prevented from getting unhooked, thereby ensuring high reliability.